Methods and systems for open loop and closed loop control of an exhaust gas recirculation system

ABSTRACT

Methods and systems are provided for estimating exhaust gas recirculation (EGR) flow in an engine including an EGR system. In one example, a method may include operating the EGR system in an open loop feed forward mode based on an intake carbon di oxide sensor output above a threshold engine load and/or when a manifold absolute pressure (MAP) is above a threshold pressure, and operating the EGR system in a closed loop feedback mode based on a differential pressure sensor output when the engine load decreases below the threshold load and/or when the MAP decreases below the threshold pressure.

FIELD

The present description relates generally to methods and systems forcontrolling an exhaust gas recirculation system of a vehicle engine.

BACKGROUND/SUMMARY

Engine control systems employ exhaust gas recirculation (EGR) mechanismsto regulate exhaust emissions and improve fuel economy. EGR mechanismsmay include an EGR system which recirculates a portion of the exhaustgas from the exhaust passage to the intake passage via an EGR passage.The EGR systems employ a delta pressure (DP) sensor across an orificelocated downstream of an EGR valve in the EGR passage to provide anestimate of EGR mass flow. The estimated EGR mass flow is then utilizedto determine a degree of spark advance.

However, at certain engine operating conditions, such as at high loadconditions, and/or when an manifold absolute pressure (MAP) is greaterthan a threshold pressure, a differential pressure across the orifice ismodulated due to pulsating flow of the exhaust. Therefore, the DP sensormay output a higher voltage due to the root mean square value of theexhaust pulsations. In other words, the exhaust pulsation may cause theDP sensor to output a higher voltage than actual. As a result, EGR massflow may be estimated to be higher than actual flow during the high loadconditions. Since spark advance is based on the estimated EGR mass flow,typically one degree of spark advance for each percent of EGR estimated,for example, an overestimation of EGR mass may lead to potential sparkknock (due to over-advanced spark timing). As a result, it may benecessary to retard the spark timing to reduce knock, which may lead toreduced fuel economy and performance.

The inventors herein have recognized the above-mentioned issues.Accordingly, in one example, some of the above issues may be at leastpartially addressed by a method for an engine, comprising: estimating anexhaust gas recirculation (EGR) mass flow based on a differentialpressure sensor output when an engine load is below a threshold;estimating the EGR mass flow based on an intake carbon dioxide sensoroutput when the engine load is above the threshold and independent ofthe differential pressure sensor output; and adjusting a spark timingbased on the estimated EGR mass flow. In this way, more accurate EGRflow estimations may be performed across various load conditions.Consequently, more accurate spark advance may be scheduled, whichreduces the chances of spark knock.

As one example, during certain engine operating conditions, such as whenan engine load is above a threshold load and/or when a MAP is above athreshold pressure, the EGR system may be operated in an open loopcontrol mode. In the open loop control mode, the EGR mass flow isestimated independent of DP sensor output but rather based on feedforward mapped intake carbon dioxide data based on engine speed andload; and a degree of spark advance is scheduled based on the EGR massflow estimated based on intake carbon dioxide values. Further, duringthe open loop mode, an EGR valve is not controlled based on DP sensoroutput but rather maintained in a fully open position or in a nearlyfully open position that is based on threshold load.

During engine operating conditions below the threshold, the EGR systemmay be operated in a closed loop control mode. In the closed loopcontrol mode, the EGR mass flow is estimated based on DP sensor output,and the degree of spark advance is scheduled based on the DP sensorbased EGR mass flow estimation. Further, during the closed loop controlmode, the EGR valve is controlled based on DP sensor output. Forexample, the EGR valve is adjusted based on an error between an actualDP sensor output and a desired DP sensor output.

In this way, by switching between open loop and closed loop control ofthe EGR system, more accurate EGR flow estimations may be performed.Consequently, more accurate spark advance may be scheduled, which maylead to reduced spark knock. As a result, unwarranted spark retard maybe reduced, resulting in improved fuel economy and performance. Thus, byutilizing open loop control and closed loop control of the EGR systembased on load and intake manifold pressure, the technical effect of moreaccurate EGR flow estimation, more accurate spark advance, and reducedspark knock may be achieved, and hence fuel economy may be improved.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of a twin-turbocharged engine system,including a high pressure EGR system.

FIG. 2 shows an example speed-load map depicting open and closed loopEGR control modes.

FIG. 3 shows an example control block diagram illustrating example openloop and closed loop EGR control.

FIG. 4 shows a flowchart illustrating an example method for switchingbetween closed loop and open loop EGR control modes.

FIG. 5 shows a flowchart illustrating an example method for closed loopEGR control.

FIG. 6 illustrates an example EGR system operation according to thepresent disclosure.

DETAILED DESCRIPTION

The following description relates to systems and methods for open loopand closed loop control of an EGR system in an engine system (such asengine system of FIG. 1) based on load and/or intake manifold pressurefor improving EGR mass flow estimation in regions of high exhaustpulsations. Specifically, as shown at FIG. 2, the EGR system may beoperated in a closed loop feedback mode at engine operating conditionsbelow a threshold load, and when the engine is operating at loads abovethe threshold load, the EGR system may be operated in an open loop feedforward mode. In some examples, additionally or alternatively, the EGRsystem may be operated in the closed loop feedback mode when a manifoldabsolute pressure (MAP) is below a threshold pressure, and when the MAPis above the threshold pressure, the EGR system may be operated in theclosed loop feedforward mode. An example of the open loop feed forwardmode and the closed loop feedback mode is illustrated in a block diagramat FIG. 3. A controller, such as the controller of FIG. 1, may beconfigured to perform a control routine, such as the example routine ofFIG. 4 for switching EGR system operation between open loop and closedloop modes, and the example routine of FIG. 5 for operating the EGRsystem in the closed loop mode. An example open and closed loop controlof the EGR system according to the present disclosure is shown at FIG.6.

Turning to FIG. 1, it shows a schematic depiction of an exampleturbocharged engine system 100 including a multi-cylinder internalcombustion engine 10 and twin turbochargers 120 and 130, which may beidentical. As one non-limiting example, engine system 100 can beincluded as part of a propulsion system for a passenger vehicle. Whilenot depicted herein, other engine configurations such as an engine witha single turbocharger, or an engine without a turbocharger may be usedwithout departing from the scope of this disclosure.

Engine system 100 may be controlled at least partially by a controller12 and by input from a vehicle operator 190 via an input device 192. Inthis example, input device 192 includes an accelerator pedal and a pedalposition sensor 194 for generating a proportional pedal position signalPP. Controller 12 may be a microcomputer including the following: amicroprocessor unit, input/output ports, an electronic storage mediumfor executable programs and calibration values (e.g., a read only memorychip), random access memory, keep alive memory, and a data bus. Thestorage medium read-only memory may be programmed with computer readabledata representing non-transitory instructions executable by themicroprocessor for performing the routines described herein as well asother variants that are anticipated but not specifically listed.Controller 12 may be configured to receive information from a pluralityof sensors 165 and to send control signals to a plurality of actuators175 (various examples of which are described herein). Other actuators,such as a variety of additional valves and throttles, may be coupled tovarious locations in engine system 100. Controller 12 may receive inputdata from the various sensors, process the input data, and trigger theactuators in response to the processed input data based on instructionor code programmed therein corresponding to one or more routines. Inother words, the controller 12 receives signals from the various sensorsof FIG. 1, including DP sensor 217, intake carbon dioxide sensor 220,manifold air pressure (MAP) sensor 182, and intake oxygen sensor 168,and employs the various actuators of FIG. 1, such as a motor actuatorfor high pressure EGR valve 210, spark timing, etc., to adjust engineoperation based on the received signals and instructions stored on amemory of the controller. Example control routines are described hereinwith regard to FIGS. 3-5.

Engine system 100 may receive intake air via intake passage 140. Asshown at FIG. 1, intake passage 140 may include an air filter 156 and anair induction system (AIS) throttle 115. The position of AIS throttle115 may be adjusted by the control system via a throttle actuator 117communicatively coupled to controller 12.

At least a portion of the intake air may be directed to a compressor 122of turbocharger 120 via a first branch of the intake passage 140 asindicated at 142 and at least a portion of the intake air may bedirected to a compressor 132 of turbocharger 130 via a second branch ofthe intake passage 140 as indicated at 144. Accordingly, engine system100 includes a low-pressure AIS system (LP AIS) 191 upstream ofcompressors 122 and 132, and a high pressure AIS system (HP AIS) 193downstream of compressors 122 and 132.

A positive crankcase ventilation (PCV) conduit 198 may couple acrankcase (not shown) to the second branch 144 of the intake passagesuch that gases in the crankcase may be vented in a controlled mannerfrom the crankcase. Further, evaporative emissions from a fuel vaporcanister (not shown) may be vented into the intake passage through afuel vapor purge conduit 195 coupling the fuel vapor canister to thesecond branch 144 of the intake passage.

The first portion of the total intake air can be compressed viacompressor 122 where it may be supplied to intake manifold 160 viaintake air passage 146. Thus, intake passages 142 and 146 form a firstbranch of the engine's air intake system. Similarly, a second portion ofthe total intake air can be compressed via compressor 132 where it maybe supplied to intake manifold 160 via intake air passage 148. Thus,intake passages 144 and 148 form a second branch of the engine's airintake system. As shown at FIG. 1, intake air from intake passages 146and 148 can be recombined via a common intake passage 149 beforereaching intake manifold 160, where the intake air may be provided tothe engine. In some examples, intake manifold 160 may include an intakemanifold pressure sensor 182 for estimating a manifold pressure (MAP)and/or an intake manifold temperature sensor 183 for estimating amanifold air temperature (MCT), each communicating with controller 12.In the depicted example, intake passage 149 also includes an air cooler154 and a throttle 158. The position of throttle 158 may be adjusted bythe control system via a throttle actuator 157 communicatively coupledto controller 12. As shown, throttle 158 may be arranged in intakepassage 149 downstream of air cooler 154, and may be configured toadjust the flow of an intake gas stream entering engine 10.

As shown at FIG. 1, a compressor bypass valve (CBV) 152 may be arrangedin CBV passage 150 and a CBV 155 may be arranged in CBV passage 151. Inone example, CBVs 152 and 155 may be electronic pneumatic CBVs (EPCBVs).CBVs 152 and 155 may be controlled to enable release of pressure in theintake system when the engine is boosted. An upstream end of CBV passage150 may be coupled with intake passage 144 upstream of compressor 132,and a downstream end of CBV passage 150 may be coupled with intakepassage 148 downstream of compressor 132. Similarly, an upstream end ofa CBV passage 151 may be coupled with intake passage 142 upstream ofcompressor 122, and a downstream end of CBV passage 151 may be coupledwith intake passage 146 downstream of compressor 122. Depending on aposition of each CBV, air compressed by the corresponding compressor maybe recirculated into the intake passage upstream of the compressor(e.g., intake passage 144 for compressor 132 and intake passage 142 forcompressor 122). For example, CBV 152 may open to recirculate compressedair upstream of compressor 132 and/or CBV 155 may open to recirculatecompressed air upstream of compressor 122 to release pressure in theintake system during selected conditions to reduce the effects ofcompressor surge loading. CBVs 155 and 152 may be either actively orpassively controlled by the control system.

As shown, a compressor inlet pressure (CIP) sensor 196 is arranged inthe intake passage 142. and an HP AIS pressure sensor 169 is arranged inintake passage 149. However, in other anticipated embodiments, sensors196 and 169 may be arranged at other locations within the LP AIS and HPAIS, respectively.

Engine 10 may include a plurality of cylinders 14. In the depictedexample, engine 10 includes six cylinders arrange in a V-configuration.Specifically, the six cylinders are arranged on two banks 13 and 15,with each bank including three cylinders. In alternate examples, engine10 can include two or more cylinders such as 3, 4, 5, 8, 10 or morecylinders. These various cylinders can be equally divided and arrangedin alternate configurations, such as V, in-line, boxed, etc. Eachcylinder 14 may be configured with a fuel injector 166. In the depictedexample, fuel injector 166 is a direct in-cylinder injector. However, inother examples, fuel injector 166 can be configured as a port based fuelinjector.

Intake air supplied to each cylinder 14 (herein, also referred to ascombustion chamber 14) via common intake passage 149 may be used forfuel combustion and products of combustion may then be exhausted fromvia bank-specific exhaust passages. In the depicted example, a firstbank 13 of cylinders of engine 10 can exhaust products of combustion viaa common exhaust passage 17 and a second bank 15 of cylinders canexhaust products of combustion via a common exhaust passage 19.

The position of intake and exhaust valves of each cylinder 14 may beregulated via hydraulically actuated lifters coupled to valve pushrods,or via mechanical buckets in which cam lobes are used. In this example,at least the intake valves of each cylinder 14 may be controlled by camactuation using a cam actuation system. Specifically, the intake valvecam actuation system 25 may include one or more cams and may utilizevariable cam timing or lift for intake and/or exhaust valves. Inalternative embodiments, the intake valves may be controlled by electricvalve actuation. Similarly, the exhaust valves may be controlled by camactuation systems or electric valve actuation. In still anotheralternative embodiment, the cams may not be adjustable.

Products of combustion that are exhausted by engine 10 via exhaustpassage 17 can be directed through exhaust turbine 124 of turbocharger120, which in turn can provide mechanical work to compressor 122 viashaft 126 in order to provide compression to the intake air.Alternatively, some or all of the exhaust gases flowing through exhaustpassage 17 can bypass turbine 124 via turbine bypass passage 123 ascontrolled by wastegate 128. The position of wastegate 128 may becontrolled by an actuator (not shown) as directed by controller 12. Asone non-limiting example, controller 12 can adjust the position of thewastegate 128 via pneumatic actuator controlled by a solenoid valve. Forexample, the solenoid valve may receive a signal for facilitating theactuation of wastegate 128 via the pneumatic actuator based on thedifference in air pressures between intake passage 142 arranged upstreamof compressor 122 and intake passage 149 arranged downstream ofcompressor 122. In other examples, other suitable approaches other thana solenoid valve may be used for actuating wastegate 128.

Similarly, products of combustion that are exhausted by engine 10 viaexhaust passage 19 can be directed through exhaust turbine 134 ofturbocharger 130, which in turn can provide mechanical work tocompressor 132 via shaft 136 in order to provide compression to intakeair flowing through the second branch of the engine's intake system.Alternatively, some or all of the exhaust gases flowing through exhaustpassage 19 can bypass turbine 134 via turbine bypass passage 133 ascontrolled by wastegate 138. The position of wastegate 138 may becontrolled by an actuator (not shown) as directed by controller 12. Asone non-limiting example, controller 12 can adjust the position ofwastegate 138 via a solenoid valve controlling a pneumatic actuator. Forexample, the solenoid valve may receive a signal for facilitating theactuation of wastegate 138 via the pneumatic actuator based on thedifference in air pressures between intake passage 144 arranged upstreamof compressor 132 and intake passage 149 arranged downstream ofcompressor 132. In other examples, other suitable approaches other thana solenoid valve may be used for actuating wastegate 138.

In some examples, exhaust turbines 124 and 134 may be configured asvariable geometry turbines, wherein controller 12 may adjust theposition of the turbine impeller blades (or vanes) to vary the level ofenergy that is obtained from the exhaust gas flow and imparted to theirrespective compressor. Alternatively, exhaust turbines 124 and 134 maybe configured as variable nozzle turbines, wherein controller 12 mayadjust the position of the turbine nozzle to vary the level of energythat is obtained from the exhaust gas flow and imparted to theirrespective compressor. For example, the control system can be configuredto independently vary the vane or nozzle position of the exhaust gasturbines 124 and 134 via respective actuators. Products of combustionexhausted by the cylinders via exhaust passage 19 may be directed to theatmosphere via exhaust passage 180 downstream of turbine 134, whilecombustion products exhausted via exhaust passage 17 may be directed tothe atmosphere via exhaust passage 170 downstream of turbine 124.Exhaust passages 170 and 180 may include one or more exhaustafter-treatment devices, such as a catalyst, and one or more exhaust gassensors. For example, as shown at FIG. 1, exhaust passage 170 mayinclude an emission control device 129 arranged downstream of theturbine 124, and exhaust passage 180 may include an emission controldevice 127 arranged downstream of the turbine 134. Emission controldevices 127 and 129 may be selective catalytic reduction (SCR) devices,three way catalysts (TWC), NO_(X) traps, various other emission controldevices, or combinations thereof. Further, in some embodiments, duringoperation of the engine 10, emission control devices 127 and 129 may beperiodically regenerated by operating at least one cylinder of theengine within a particular air/fuel ratio, for example.

Engine system 100 may further include one or more exhaust gasrecirculation (EGR) systems for recirculating at least a portion ofexhaust gas from the exhaust manifold to the intake manifold. These mayinclude one or more high pressure EGR systems for proving high pressureEGR (HP EGR) and one or more low-pressure EGR-loops (not shown) forproviding low pressure EGR (LP EGR).

In the depicted example, the engine system 100 may include a HP EGRsystem 206. In the depicted example, engine system 100 may be equippedwith HP EGR system on only one bank of the V engine. HP EGR system 206routes a desired portion of exhaust gas from common exhaust passage 17,upstream of the turbine 124, to intake manifold 160, downstream ofintake throttle 158. Alternatively, the HP EGR system 206 may bepositioned between exhaust passage 17 and the intake passage 193,downstream of the compressor 122. The amount of HP EGR provided tointake manifold 160 may be varied by the controller 12 via EGR valve 210coupled in the HP EGR passage 208. For example, controller 12 may adjusta position of EGR valve 210 by sending a control signal (e.g., anelectrical signal such as voltage or current) to an actuator of the EGRvalve, such as a motor, which may be a dc motor, to provide a desiredamount of HP EGR. In the example embodiment shown at FIG. 1, HP EGRsystem 206 includes an EGR cooler 212 positioned upstream of EGR valve210. EGR cooler 212 may reject heat from the recirculated exhaust gas toengine coolant, for example.

Further, a differential pressure (DP) sensor 217 may be arranged withinthe EGR passage. DP sensor 217 may provide an indication of one or moreof pressure, temperature, and concentration of the exhaust gas. In oneexample, output from DP sensor 217 may be utilized with output from MAPsensor 182 to estimate an EGR mass flow. For example, DP sensor 217 maydetect a pressure drop across an EGR flow control orifice 219 placeddownstream of EGR valve 210, which when combined with MAP, can providean indication of the amount of EGR. As such, DP sensor 217 may provide adifferential pressure signal DP across the EGR orifice to the controller12. In some examples, sensor 217 may comprise a manifold absolutepressure (MAP) sensing element and a differential pressure (DP) sensingelement, as well as a manifold temperature sensing element. In someexamples, the flow control orifice 219 may be placed upstream of EGRvalve 210, or downstream of EGR valve 210 as shown. In some examples, anadditional sensor may sense EGR valve position to indicate EGR valveflow area changes based upon commands from controller 12 and thusprovide feedback control for valve position control.

Engine system 100 may also include a second high pressure EGR loop (notshown) for recirculating at least some exhaust gas from the exhaustpassage 19, upstream of the turbine 134, to the intake passage 148,downstream of the compressor 132, or to the intake manifold 160,downstream of intake throttle 158. EGR flow through HP EGR loop 208 maybe controlled via HP EGR valve 210.

EGR valve 210 may be configured to adjust an amount and/or rate ofexhaust gas diverted through the corresponding EGR passages to achieve adesired EGR dilution percentage of the intake charge entering theengine. As a specific example, during engine operation at lower loadconditions below a threshold load, EGR valve 210 may be controlled basedon a closed-loop feedback mechanism to achieve a desired EGR mass flow.For example, when the engine load is below the threshold, output from DPsensor 217 may be utilized to estimate an actual EGR mass flow (that is,EGR mass flow at a given time). Based on engine operating conditions(e.g., engine speed and engine load), a desired EGR mass flow may bedetermined. The engine controller may then determine an error betweenthe actual and the desired EGR mass flow, and adjust EGR valve 210 basedon the error. Further, based on the actual EGR mass flow determinedbased on the DP sensor output, a degree of spark advance may bescheduled. In some examples, an error between an actual (or measured) DPsensor output and a desired DP sensor output may be utilized by thecontroller to adjust EGR valve 210.

In this way, when operating at loads below the threshold load, feedbackfrom the DP sensor may be utilized to control the EGR valve to providedesired EGR mass flow. In other words, during engine operation below thethreshold load, the EGR system is operated in a closed loop mode withfeedback from DP sensor.

However, when operating at higher loads above the threshold load,pulsating exhaust flow may cause the DP sensor to indicate a higher EGRmass flow than the actual EGR mass flow. For example, due to the rootmean square values of the exhaust pulsations, the DP sensor may output ahigher voltage than actual voltage. Consequently, the DP sensor mayindicate a higher EGR mass flow than actual. Therefore, in order toimprove accuracy of EGR flow estimation, when the engine is operating athigher loads above the threshold, the actual EGR mass flow at a giventime may be determined independent of the DP sensor output but ratherinferred from an output of an intake carbon di oxide sensor 220. Forexample, a map correlating intake carbon dioxide levels with EGRpercentage at various speed and load conditions may be stored within amemory of the controller. Then, based on the intake carbon dioxideconcentration measured by the intake carbon di oxide sensor and engineair flow measured by a MAF sensor, the EGR mass flow may be determined.Then, the degree of spark advance is determined based on the EGR flowestimated based on the intake carbon di oxide sensor.

Further, during engine operation at loads above the threshold, due toexhaust pulsation corrupting DP sensor output as discussed above, theengine controller may not adjust the EGR valve based on DP sensor outputbut rather an open loop control mechanism of the EGR system may beemployed. That is, the EGR valve may be maintained at an open loopposition when the engine is operating above the threshold load and theactual EGR mass flow is estimated based on the intake carbon di oxidesensor. In one example, the open loop position may be a fully openposition of EGR valve 210. In another example, the open loop positionmay be based on the threshold load. For example, at the threshold load,the controller may adjust the EGR valve at a threshold load position toprovide a desired EGR flow. Then, as long as the load remains at orabove the threshold load, the EGR valve may be maintained at thethreshold load position, and the actual EGR mass flow may be estimatedbased on the mapped intake carbon dioxide values.

In some examples, the threshold load may be based on a fully openposition of EGR valve 210. In such cases, the threshold load positionmay be the fully open position. In some examples, the threshold loadposition may be a nearly fully open position.

In the depicted example, the intake carbon di oxide sensor is positionedat a junction of intake passage 149 and HP EGR passage 208. However, inother embodiments, the intake carbon dioxide sensor may be positionedwithin intake passage 149 downstream of throttle 158. The intake carbondioxide sensor may be any suitable sensor for providing an indication ofcarbon dioxide concentration in the intake charge.

In this way, when operating at loads at or above the threshold, feedbackfrom the DP sensor may not be utilized to control the EGR valve; ratherthe EGR valve may be maintained at a specific position corresponding tothe threshold load or the EGR valve may be fully opened, and the EGRflow estimation for scheduling spark may be based on mapped intakecarbon dioxide values. In other words, during engine operation below thethreshold load, the EGR system is operated in an open loop mode withoutfeedback from DP sensor but rather based on intake carbon di oxidesensor output.

While the above example describes switching between open loop and closedloop operation of the EGR system responsive to load, the inventorsherein have further identified that exhaust pulsations cause DP sensormeasurement errors when a manifold absolute pressure (MAP) determinedbased on MAP sensor 182 is above a threshold pressure (such as 26 inchesHg). Therefore, EGR system operation may switch between closed loop andopen loop operation responsive to MAP. Accordingly, in one example,during engine operation below the threshold load and/or when MAP isbelow the threshold pressure, the EGR system may be operated in theclosed loop mode; and during engine operation above the threshold loadand/or when MAP is above the threshold pressure, the EGR system may beoperated in the open loop mode.

Thus, by switching between open loop and closed loop mode of operationof the EGR system, the effect of exhaust pulsation on EGR flowestimation may be reduced. Therefore, more accurate EGR flow estimationsmay be obtained, leading to more accurate spark advance scheduling. As aresult, spark knock may be reduced, which leads to improved fuel economyand performance.

Further, while the example depicted herein shows a boosted enginesystem, it must be noted that the switching between open loop and closedloop operation of the EGR system to reduce the effect of exhaustpulsation on EGR flow estimation may be applied in other engineconfigurations, such as a naturally aspirated engine, without departingfrom the scope of this disclosure,

An example graph depicting engine speed along X-axis and engine loadalong Y-axis, and indicating regions of open loop and closed loopoperation of the EGR system is shown at FIG. 2. Specifically, FIG. 2shows operation of the EGR system in the open loop mode at loads abovethreshold 202, and operation of the EGR system in the closed loop modeat loads below the threshold 202. While the example shown in FIG. 2indicates regions of open loop and closed loop EGR system control in aspeed-load map, it must be noted that the EGR system operation may beadditionally or alternatively based on MAP as discussed briefly above.For example, the EGR system may be operated in the open loop mode whenthe engine load is above the threshold load 202 and/or when MAP(determined based on MAP sensor 182, for example) is above a thresholdpressure; otherwise the EGR system may be operated in the closed loopmode. Details of operating the EGR system in the open loop mode and theclosed loop mode, and switching between the two modes are furtherdescribed below with respect to FIGS. 3-6.

While the above examples illustrate estimating EGR mass flow during openloop control mode based on the intake carbon dioxide sensor, it will beappreciated that in some examples, an intake oxygen sensor 168 may beutilized for EGR mass flow estimation. In the depicted example, theintake oxygen sensor is positioned downstream of air cooler 154.However, in other embodiments, sensor 168 may be arranged at a junctureof intake passages 146, 148, and 149 and upstream of air cooler 154 orat another location along intake passage 149, such as downstream ofthrottle 158. Intake oxygen sensor (IAO2) 168 may be any suitable sensorfor providing an indication of the oxygen concentration of the intakecharge, such as a linear oxygen sensor, intake UEGO (universal orwide-range exhaust gas oxygen) sensor, two-state oxygen sensor, etc.Controller 12 may estimate the percent dilution of the EGR flow based onfeedback from intake oxygen sensor 168. In some examples, the controllermay then adjust one or more of EGR valve 121, AIS throttle 115, or otheractuators to achieve a desired EGR dilution percentage of the intakecharge.

Engine system 100 may include various sensors 165, in addition to thosementioned above. As shown in FIG. 1, common intake passage 149 mayinclude a throttle inlet pressure (TIP) sensor 172 for estimating athrottle inlet pressure (TIP) and/or a throttle inlet temperature sensor173 for estimating a throttle air temperature (TCT), each communicatingwith controller 12. Further, while not depicted herein, each of intakepassages 142 and 144 can include a mass air flow sensor or alternativelythe mass air flow sensor can be located in common duct 140.

Turning to FIG. 3, an example method 300 for closed loop and open loopoperation of an EGR system, such as EGR system 206 at FIG. 1, is shownin a block diagram form. In particular, method 300 includes, at loadsabove a threshold, operating the EGR system in an open loop mode 330,where an EGR flow estimation is based on mapped intake carbon dioxidevalues; and at loads below the threshold, operating the EGR system in aclosed loop mode 320 based on a pressure feedback mechanism for EGR flowcontrol by controlling the EGR valve through a PID controller.

When the engine load is below the threshold, the controller determines adesired EGR percentage of fresh airflow (%EGR) based on engine speed andload. Then, based on the engine air mass (as determined from MAP andspeed density calculations, or a mass airflow sensor), at 306, thecontroller determines a desired EGR mass (DES EM).

Next, at 308, a desired differential pressure (DES DP) may be determinedbased on the desired EGR mass and the measured MAP from block 310. Block310 may contain calculated MAP based on pressures detected by a DPsensor, such as sensor 217 at FIG. 1, located at the intake manifold.Next, at 312, a desired DP sensor voltage (DES VOLTAGE) may becalculated based on DES DP. Then, at 313, an error signal (ERROR) may becalculated based on DES VOLTAGE from block 312 and an actual DP sensorvoltage (block 314) from the DP sensor. A PID controller, shown at block316 may then determine an actuation signal based on calculated ERRORsignal. The actuation signal may be used to adjust EGR flow bycontrolling the EGR valve, such as EGR valve 210, through the PIDcontroller. For example, by supplying the actuation signal to a motoractuator, such as actuator 340, a position of the EGR valve may beadjusted to a desired position to provide the desired EGR mass (DES EM)to the engine. The actuation signal may be a duty cycle or a voltagesignal, for example.

Further, during closed loop control, an actual EGR flow may be estimatedbased on the actual DP sensor voltage, and a degree of spark advance maybe scheduled based on the actual EGR flow estimated based on the actualDP sensor voltage.

In this way, during closed loop mode, EGR flow may be adjusted based ona pressure feedback mechanism from the DP sensor, which includescontrolling the EGR flow through the EGR valve based on an errorcalculated between desired sensor voltage and actual sensor voltage. Insome examples, an error between the actual EGR mass flow determinedbased on the actual sensor voltage and the desired EGR mass flow (DESEM)may be used to provide feedback control of the EGR valve.

When the engine load is above the threshold, the EGR system may beoperated in the open loop control mode, indicated at 330. During theopen loop mode, the controller may adjust the EGR valve to a fully openposition and as indicated at 332, the actual EGR mass flow may beestimated based on intake carbon dioxide levels measured by a carbondioxide sensor/analyzer, such as sensor 220 shown at FIG. 1, locatedwithin the intake manifold. For example, intake carbon dioxide levelsmay be mapped with EGR percentage at various speed and load conditions,and stored in a look-up table or a map in a memory of the controller.Then, based on the measured/estimated intake carbon dioxide levels, anEGR percentage may be calculated. Subsequently, the actual EGR mass flowmay be determined as a function of EGR percentage and engine air mass.Then, at 334, based on the actual EGR mass flow, a degree of sparkadvance may be scheduled.

In one example, in response to a load change from a first load below athreshold to a second load greater than the threshold, transitioningfrom a closed loop EGR flow control to an open loop EGR flow control,wherein during the transition the EGR valve is ramped open to a fullyopen position. Further, during the transition, as the EGR valve isramped open, the EGR valve may be assumed to be in a fully open positionand the EGR mass flow may be estimated based on intake carbon dioxidelevels as measured by the carbon dioxide sensor located within theintake manifold. For example, a look-up table mapping intake carbondioxide levels with EGR percentage for open loop control may be storedin a memory of the controller. Upon estimating the intake carbon dioxidelevels, the controller may estimate EGR percentage based on the look-uptable. The estimated EGR mass flow may then be calculated based on theestimated EGR percentage, and a degree of spark advance may be scheduledbased on the estimated EGR mass. In this way, during high loadconditions, by estimating EGR flow based on intake carbon dioxide levelsand independent of DP sensor output, errors in EGR flow estimation dueto exhaust pulsations that affect DP sensor output may be reduced.

Further, in response to a load change from the second load above thethreshold to a first load below the threshold, transitioning from openloop EGR flow control to closed loop EGR flow control, wherein duringthe transition to the closed loop EGR flow control, the EGR valve isramped from a fully-open position to a more closed position. Theactuation signal for adjusting the EGR valve to a more closed positionis based on a desired EGR flow that is determined based on current speedand load condition. However, for scheduling spark advance during thetransition period, as the EGR valve is ramped to the more closedposition, the EGR flow may be determined based on intake carbon dioxidelevels and the spark advance may be adjusted based on the intake carbondioxide levels. Upon completing the transition, the spark advance may bescheduled based on EGR mass flow rate determined based on DP sensoroutput.

In this way, operation of the EGR system may be switched between an openloop mode and closed loop mode based on load to obtain a more accurateestimation of EGR mass flow.

While the above example illustrates EGR system operation based on loadalone, in one example, when the load is below the threshold and/or whena MAP (based on output from a MAP sensor, such as sensor 182 at FIG. 1)is below a threshold pressure, the EGR system may be operated in theclosed loop mode; and when the load is above the threshold and/or whenthe MAP is above the threshold pressure, the EGR system may be operatedin the open loop mode. In one example, the threshold pressure may be 26inches Hg.

Turning to FIG. 4, a flow chart illustrating an example method 400 foroperating an EGR system, such as EGR system 206 at FIG. 1, including anEGR valve, such as valve 210 at FIG. 1, in an engine, such as engine 10at FIG. 1, is shown. Specifically, method 400 illustrates switchingbetween closed loop and open loop operation of the EGR system. Whilemethod 400 illustrates closed loop and open loop operation of the EGRsystem with respect to boosted engine system depicted at FIG. 1, it mustbe noted that method 400 and the rest of the methods included herein maybe applicable to other engine systems, such as a naturally aspiratedengine, without departing from the scope of the present disclosure.Further, instructions for carrying out method 400 and the rest of themethods included herein may be executed by a controller, such ascontroller 12 shown at FIG. 1, based on instructions stored on a memoryof the controller and in conjunction with signals received from sensorsof the engine system, such as the sensors described above with referenceto FIG. 1. The controller may employ engine actuators of the enginesystem to adjust engine operation, according to the methods describedbelow.

Method 400 begins at 402. At 402, method 400 includes estimating and/ormeasuring engine operating conditions. These may include, for example,engine speed and load, driver torque demand (based on accelerator pedalposition), boost, MAP, MAF, BP, engine temperature, EGR mass flow,air-fuel ratio, etc. Based on engine operating conditions and torquedemand, the vehicle controller may adjust one or more engine actuatorsettings. The actuator settings adjusted may include, for example, sparktiming, EGR valve opening, variable cam timing (VCT), AFR, throttleopening, etc.

Next, at 404, method 400 includes determining if an engine load isgreater than a threshold load. In some examples, additionally oralternatively, it may be determined is MAP is greater than a thresholdpressure. In one example, the threshold may correspond to a load in thehigher load range. For example, the threshold load may be 9 bar brakemean effective pressure (BMEP). In another example, the threshold may bebased on a fully opened position of the EGR valve. At the threshold loadand above, due to exhaust pulsations in the turbocharged engine, adifferential pressure (DP) sensor, such as sensor 217 at FIG. 1,utilized to estimate an EGR mass flow may output a higher voltageleading to a higher EGR flow estimate than actual EGR flow. Thus, whenthe engine is operating at or above the threshold engine load, the EGRsystem may be operated in an open loop mode, which does not rely on DPsensor output for EGR flow estimation but rather, the EGR flow isestimated based on intake carbon dioxide sensor, such as sensor 220 atFIG. 1, as discussed below. Further, in the example method 400 discussedherein, the EGR valve may be maintained in a fully open position whenoperating in the open loop mode.

Accordingly, if the engine load is greater than the threshold, method400 proceeds to 406. At 406, method 400 includes determining if the EGRvalve is fully open. For example, when operating at a first engine loadbelow a threshold, prior to a transition from the first engine load to asecond engine load at or above the threshold, the engine may beoperating in the closed loop mode for EGR flow control. Consequently,prior to the transition, the EGR valve position may be adjusted based onfeedback from the DP sensor to provide a desired EGR flow. Thus, priorto the transition, the EGR valve may not be fully open, and when theload changes from the first load to the second load, the EGR valve maynot be fully open. Accordingly, if it is determined that the EGR valveis not fully opened at a time when the load is above the threshold,method 400 proceeds to 408.

At 408, method 400 includes ramping the EGR valve to a fully openposition. For example, ramping the EGR valve to the fully open positionmay include adjusting the EGR valve from a less open position to anintermediate more open position and finally, to a fully open position.The transition from the less open position to the fully open positionmay include one or more intermediate more open positions. In someexamples, such as when an EGR valve opening amount at the less openposition is greater than a threshold opening amount, the EGR valve maytransition from the less open position to the fully open positionwithout any intermediate more open positions. Further, during theramping, spark may be scheduled based on EGR flow estimated on mappedintake carbon dioxide values. Upon ramping open the EGR valve to thefully open position, method 400 proceeds to 412.

Retuning to 406, if it is determined that the EGR valve is fully opened,method 400 proceeds to 410. At 410, method 400 includes maintaining theEGR valve at the fully open position. The EGR valve may be maintained atthe fully open position as long as the load remains at or greater thanthe threshold.

While in this example method 400, the EGR valve is ramped open to afully open position, it must be noted that in some examples, when theload increases above the threshold, the EGR valve may be ramped open toa threshold load position which may not be a fully open position, andmaintained at the threshold load position as long as the load remainsabove the threshold. The threshold load position may be a nearly fullyopen position, for example.

Next, method 400 proceeds to 412. At 412, method 400 includes estimatingEGR mass flow (414) based on mapped intake carbon dioxide values andindependent of DP sensor output while maintaining the EGR valve in thefully open position. That is, the EGR system is operated in an open loopmode feed-forward mode, wherein, the EGR valve is maintained in thefully open position until the load decreases below the threshold, andwhile the EGR valve is the fully open position, the EGR mass flow isestimated based on an output from the carbon dioxide sensor, engine loadand engine speed, and mass air flow. For example, the EGR mass flow maybe estimated based on a look-up table that maps intake carbon dioxidevalues to EGR percentage based on engine speed and engine load. Based onestimated EGR percentage from mapped carbon dioxide values, and mass airflow, the EGR mass flow may be estimated. Further, at 414, method 400includes turning off the EGR system diagnostics (416) during EGRoperation in the open loop mode.

Upon estimating the EGR mass flow, method 400 proceeds to 418. At 418,method 400 includes scheduling spark based on the estimated EGR mass. Inthis way, by switching to open loop feed-forward control of the EGR flowwhen the engine is operating at or above the load threshold, andscheduling spark based on estimated EGR flow from mapped intake carbondioxide values rather than DP sensor output, potential spark knock maybe reduced. Consequently, unwarranted spark retard may be reduced, whichmay lead to improved fuel economy and performance.

In one example, switching from closed loop feedback control to open loopfeed-forward control of the EGR flow may be performed responsive to MAPreaching or increasing above the threshold pressure. In one example, thethreshold pressure may be based on a MAP at which the EGR valve is fullyopen. In another example, the threshold pressure may be a predeterminedvalue, such as 26 inches Hg.

Returning to 404, if it is determined that the load is not greater thanthreshold, method 400 proceeds to 420. At 420, method 400 includesdetermining if EGR flow control is operated in the open loop mode. Forexample, EGR flow control may be determined to be operating in the openloop mode if the EGR valve is not adjusted responsive to DP sensoroutput and if EGR flow estimation is based on mapped intake carbondioxide values and not DP sensor output. In some examples, additionallyor alternatively, EGR flow control may be determined to be operating inthe open loop mode if an open loop indicator flag is set.

If the answer at 420 is YES, method 400 proceeds to 422. For example,when operating at a third engine load at or above the threshold, priorto a transition from the third engine load to a fourth engine load belowthe threshold, the engine may be operating in the open loop mode for EGRflow control. Thus, when the load changes from the third load to thefourth load, the EGR flow control may be in the open loop mode.Accordingly, if it is determined that the EGR flow control is operatingin the open loop mode when the load is below the threshold, method 400proceeds to 422. At 422, method 400 includes determining a desired EGRflow based on current engine speed and engine load.

Upon determining the desired EGR flow, method 400 proceeds to 424. At424, method 400 includes ramping the EGR valve from the fully openposition to a desired open position, the desired open position based onthe desired EGR flow. For example, the desired open position may be aless open position, and therefore the EGR valve may be ramped close fromthe fully open position to a less open position. During the ramping,until the desired EGR valve position is reached, the EGR system maycontinue to operate in the open loop mode. Accordingly, during theramping, the spark may be scheduled based on EGR estimation from mappedintake carbon dioxide levels.

Next, upon adjusting the EGR valve to the desired position, method 400proceeds to 426. At 426, the EGR flow control is operated in a closedloop feedback mode. Operating in the closed loop feedback mode includesadjusting the EGR valve based on feedback from the DP sensor to providean estimation of actual EGR flow and adjusting spark based on actual EGRflow estimation from the DP sensor output. Details of operating the EGRsystem in a closed loop configuration will be further elaborated withrespect to FIG. 5.

Turning to FIG. 5, a flow chart illustrating an example method 500 foroperating the EGR system in a closed loop mode is shown.

At 502, method 500 includes determining a desired exhaust gasrecirculation percentage (EGR %) based on engine speed and load. Next,at 504, method 500 includes calculating a desired EGR flow (DESEM) basedon desired EGR percentage and engine air flow (as determined from MAPand speed density calculations or a mass airflow sensor). The desiredEGR flow may be calculated according to the equation:

Desired EGR flow (DESEM)=engine air flow * EGR %/(1-EGR %)

Next, at 506, method 500 includes measuring or estimating an actual EGRflow based on output from a DP sensor, such as sensor 217 at FIG. 1. Forexample, a differential pressure across an EGR flow control orifice,such as orifice 219 at FIG. 1, is determined from the DP sensor output.Then, actual EGR flow may be determined as a function of differentialpressure and MAP, for example based on a square root of the product.Then, spark is scheduled based on the actual EGR flow.

Next, at 510, method 500 includes determining an error between theactual EGR flow and the desired EGR flow. In one example, aproportional-integral-derivative (PID) controller may be utilized tocalculate the error. Various other control architectures can be used,such as a proportional controller, or a proportional integralcontroller, or various other controllers including feedback and feedforward combined control action.

Next, at 512, method 500 includes adjusting an EGR valve, such as valve210 at FIG. 1, based on the error. For example, an actuation signalbased on calculated ERROR signal. The actuation signal may be used toadjust EGR flow by controlling the EGR valve. In particular, theactuation signal may be sent by the controller to a DC motor actuator,such as actuator 340 at FIG. 3, controlling the EGR valve.

In this way, the EGR system may be operated in a closed loop feedbackmode during engine operation at loads below the threshold.

Turning to FIG. 6, an example map 600 illustrating example EGR flowcontrol is shown. Specifically, FIG. 6 illustrates switching betweenopen loop control and closed loop control of an EGR system, such as EGRsystem 206 at FIG. 1, including an EGR valve, such as EGR valve 210 atFIG. 1, responsive to load, according to the present disclosure.

The sequence of FIG. 6 may be provided by executing instructions in thesystem of FIG. 1 according to the method of FIG. 4 in cooperation withthe method of FIG. 5. Vertical markers at times t0-t4 represent times ofinterest during the sequence.

Specifically, the first plot from the top of FIG. 6 depicts load versustime, and the load increases in the direction of the Y axis arrow. Trace602 depicts change in load and horizontal line 604 depicts thresholdload.

The second plot from the top of FIG. 6 depicts an EGR system controlmode. Trace 606 depicts operation of the EGR system in an open loop modeor a closed loop mode.

The third plot from the top of FIG. 6 depicts EGR valve position versustime, and an EGR valve opening increases in the direction of Y axisarrow. Trace 608 depicts actual change in EGR valve opening.

The fourth plot from the top of FIG. 6 depicts a status of EGR systemdiagnostics. Trace 610 depicts the status of EGR system diagnostics.

At t0, and between t0 and t1, the engine may be operating at a loadbelow the threshold. Accordingly, the EGR system may be operating in aclosed loop mode. Consequently, the EGR valve may be adjusted based onfeedback from a DP sensor, such as sensor 217 at FIG. 1. That is, asdiscussed at FIG. 3, during closed loop control, the EGR valve may beadjusted based on an error between an actual DP sensor voltage and adesired DP sensor voltage, where the desired DP sensor voltage is basedon a desired EGR flow estimated as a function of engine mass air flowand desired EGR percentage at current speed and load conditions.Further, when the load is below the threshold and the EGR system isoperating in the closed loop mode, an actual EGR flow may be calculatedbased on the actual DP sensor voltage, and the spark may be scheduledbased on the actual EGR flow determined. Further, during times at t0 andbetween t0 and t1, EGR system diagnostics may be performed if entryconditions are met.

At t1, the engine load reaches the threshold. Responsive to the loadreaching the threshold, between t1 and t2, the EGR system operation mayswitch to open loop control mode from the closed loop control mode.Switching to open loop control includes ramping the EGR valve from aless open position to a fully open position. In some examples, switchingto open loop control includes ramping the EGR valve to a threshold loadposition, which may or may not be a fully open position. Switching toopen loop control further includes, estimating the actual EGR flow basedon intake carbon dioxide levels rather than DP sensor output (e.g., DPsensor voltage). Specifically, during the switching, as the EGR valve isramped open, EGR flow may be estimated based on a map correlating intakecarbon dioxide levels at various engine speed and load with EGRpercentage. The intake carbon dioxide levels may be measured based on anoutput from a carbon dioxide sensor, such as sensor 220 at FIG. 1.Further, during the switching to open loop control mode, as the EGRvalve ramps open to the fully open position, spark may be scheduledbased on the actual EGR flow estimated based on mapped intake carbondioxide levels.

At t2, and between t2 and t3, the engine load remains above thethreshold. As a result, the EGR system is operated based on the openloop control. Operating the EGR system in the open loop control modeincludes maintaining the EGR valve in a fully open position, andestimating the actual EGR flow based on mapped intake carbon dioxidelevels. In some examples, the EGR valve may be maintained at thethreshold load position. Further, during the open loop control, thespark is scheduled based on the actual EGR flow estimated based onmapped intake carbon dioxide levels.

In this way, at high load conditions (e.g., above 9 bar BMEP), byestimating the actual EGR flow based on intake carbon dioxide levels andnot based on DP sensor output, errors in actual EGR flow estimation,which occur due to exhaust pulsations causing the DP sensor to output ahigher voltage than actual voltage, may be reduced. Consequently, excessspark advance due to overestimation of the actual EGR flow by DP sensoroutput may be reduced. As a result, spark knock may be reduced.

At t3, the engine load decreases below the threshold. In response to theengine load decreasing below the threshold, the EGR system operation isswitched to closed loop control. Accordingly, at t3, and between t3 andt4, a desired EGR flow is calculated based on engine speed and loadconditions, and the EGR valve is adjusted to a first desired position toprovide the desired EGR flow. For example, as discussed above withrespect to FIG. 3, the first desired position may be based on a desiredsensor voltage determined from a desired differential pressure, wherethe desired differential pressure is determined using a graphicalrepresentation from a desired EGR flow. The desired EGR flow may becalculated as a function of a desired EGR percentage and engine airflow, where the desired EGR percentage is based on engine speed andload. Upon calculating the desired sensor voltage, a control signal issent to an actuator of the EGR valve, to adjust the EGR valve to thefirst desired position from the fully open position. The control signalmay be a voltage level or a duty cycle, for example.

Further, adjusting the EGR valve from the fully open position to thefirst desired position includes ramping the EGR valve from the fullyopen position to the desired position. For example, if the first desiredposition is a more closed position, the EGR valve may be ramped from thefully open position to an intermediate less closed position and thenramped to the more close position. The transition from the fully openposition to the more closed (or less open) position may include one ormore intermediate less closed (or more open) positions. Further, duringramping from the fully open position to the first desired position,until the first desired position is reached, the actual EGR flow iscalculated based on intake carbon dioxide levels and the spark isscheduled based on the actual EGR flow calculated based on intake carbondioxide levels.

Taken together, during switching from the open loop control mode to theclosed loop control mode, the EGR valve is adjusted from the fully openposition to the first desired position, where the first desired positionis estimated based on the desired differential pressure sensor voltagefor the desired EGR flow at current speed and load conditions. Further,during the switching, until the EGR valve is adjusted to the firstdesired position, the actual EGR flow is estimated from the intakecarbon dioxide levels and the spark is scheduled based on the actual EGRflow estimated from intake carbon dioxide levels.

Further, between t1 and t4, when the EGR flow estimation is based on theintake carbon di oxide sensor and the EGR system is operated in an openloop mode, EGR system diagnostics may not be performed (e.g., EGR valve,DP sensor, or EGR cooler diagnostics may not be performed).

Next, at t4, the EGR valve is at the first desired position. Further, att4 and beyond t4, the engine load remains below the threshold.Accordingly, at t4 and beyond t4, the EGR system is operated in theclosed loop control mode and therefore, the actual EGR flow iscalculated based on DP sensor output and the spark is scheduled based onthe EGR flow calculated based on the DP sensor output. Further, betweent4 and beyond, the position of the EGR valve is adjusted based on anerror between the actual DP sensor voltage and desired DP sensorvoltage, where the desired DP sensor voltage is based on a desired EGRflow at current speed and load conditions. In other words, during closedloop-control, feedback from the DP sensor is utilized for control of EGRvalve position.

In this way, by switching operation of the EGR system between an openloop control mode and a closed loop control mode depending on engineload conditions, more accurate EGR flow estimation may be performed.Consequently, more accurate spark scheduling may be performed. As aresult, spark knock, due to excess spark advance from excess EGR flowestimation, for example, may be reduced. Therefore, spark retardmeasures to counteract spark knock may be reduced, leading to improvedfuel economy and efficiency.

While the present example illustrates switching between open loop andclosed loop operation of the EGR system responsive to load, EGR systemoperation may switch between closed loop and open loop operationresponsive to manifold absolute pressure (MAP). Accordingly, in oneexample, during engine operation below the threshold load and/or whenMAP is below the threshold pressure, the EGR system may be operated inthe closed loop mode; and during engine operation above the thresholdload and/or when MAP is above the threshold pressure, the EGR system maybe operated in the open loop mode. Further, the EGR system operation inclosed loop mode and open loop mode and the transition between the twomodes as discussed herein may be applied to various engineconfigurations, such as a boosted engine with high pressure and/or lowpressure EGR, and naturally aspirated engine.

In one example, a method for an engine, includes estimating an exhaustgas recirculation (EGR) mass flow based on a differential pressuresensor output when an engine load is below a threshold; estimating theEGR mass flow based on an intake carbon dioxide sensor output when theengine load is above the threshold and independent of the differentialpressure sensor output; and adjusting a spark timing based on theestimated EGR mass flow. In a first example of the method, an EGR valveis in a fully open position when the engine load is above the threshold.A second example of the method optionally includes the first example andfurther includes turning off EGR system diagnostics when the engine loadis above the threshold. A third example of the method optionallyincludes one or more of the first and second examples, and furtherincludes during a first transition from a first load below the thresholdto a second load above the threshold, estimating the exhaust gasrecirculation mass flow independent of the differential pressure sensoroutput and based on the intake carbon dioxide sensor output. A fourthexample of the method optionally includes one or more of the firstthrough third examples, and further includes wherein, during thetransition, an EGR valve is ramped open to a fully open position. Afifth example of the method optionally includes one or more of the firstthrough fourth examples, and further includes during a second transitionfrom the second load above the threshold to the first load below thethreshold, estimating the exhaust gas recirculation mass flowindependent of the differential pressure sensor output and based on theintake carbon dioxide sensor output. A sixth example of the methodoptionally includes one or more of the first through fifth examples, andfurther includes wherein an EGR valve is ramped close from a fully openposition to a less open position, the less open position based on adesired EGR flow at the second load. A seventh example of the methodoptionally includes one or more of the first through sixth examples, andfurther includes wherein the EGR is a high pressure EGR (HP EGR). Aneighth example of the method optionally includes one or more of thefirst through seventh examples, and further includes wherein the engineis a V-engine; and wherein the HP EGR is on only one bank of the engine.

In another example, a method for an engine includes in response to amanifold absolute pressure increasing above a threshold pressure,ramping an EGR valve to a fully open position; and adjusting spark basedon an intake carbon dioxide sensor output. In a first example of themethod adjusting spark based on the intake carbon di oxide sensor outputincludes determining an EGR percentage based on a mapped data, themapped data correlating EGR percentage with the intake carbon di oxidesensor output at various engine speed and load conditions; determiningan EGR flow based on the EGR percentage and engine air mass; andscheduling a degree of spark advance based on the EGR flow. A secondexample of the method optionally includes the first example and furtherincludes in response to the MAP decreasing below the threshold pressure,ramping the EGR valve from the fully open position to a desiredposition, the desired position based on engine speed and load; andadjusting spark based on an output from a differential pressure (DP)sensor measuring differential pressure across an orifice in a EGRpassage. A third example of the method optionally includes one or moreof the first and second examples, and further includes wherein thedesired position is a more closed position; and wherein a degree ofopening of the EGR valve in the more closed position is based on theengine speed and load. A fourth example of the method optionallyincludes one or more of the first through third examples, and furtherincludes wherein adjusting spark based on the differential pressure (DP)sensor output comprises: determining an EGR flow based on thedifferential pressure sensor output; and scheduling a degree of sparkadvance based on the EGR flow. A fifth example of the method optionallyincludes one or more of the first through fourth examples, and furtherincludes when the MAP is above the threshold pressure, maintaining theEGR valve in the fully open position; and when the MAP is below thethreshold pressure, controlling the EGR valve based on an error betweenthe DP sensor output and a desired sensor output, the desired sensoroutput based on a desired EGR flow at current engine speed and load.

In another example, a method for an engine includes during a firstcondition, operating a EGR valve based on an error between a desired DPsensor output and a measured DP sensor output, and adjusting sparktiming based on the measured DP sensor output; and during a secondcondition, operating the EGR valve independent of the DP sensor output,and adjusting spark timing based on an intake carbon di oxide sensorvoltage. In a first example of the method the first condition includesan engine load below a threshold, and the second condition includes anengine load above the threshold, the threshold load based on the EGRvalve in a fully open position. A second example of the methodoptionally includes the first example and further includes during afirst transition event from the first condition to the second condition,ramping the EGR valve to the fully open position while adjusting sparktiming based on an assumed fully open EGR valve position and the intakecarbon di oxide sensor voltage; and wherein operating the EGR valveindependent of the DP sensor output includes operating the EGR valve inthe fully open position. A third example of the method optionallyincludes one or more of the first and second examples, and furtherincludes during a second transition event from the second condition tothe first condition, ramping the EGR valve from a fully open position toa less open position based on a desired EGR while adjusting spark timingbased on the assumed fully open EGR valve position and the intake carbondi oxide sensor voltage; and turning off EGR system diagnostics duringthe second condition and during the first and the second transitionevents. A fourth example of the method optionally includes one or moreof the first through third examples, and further includes wherein thefirst condition includes a manifold absolute pressure (MAP) below athreshold pressure, and the second condition includes the MAP above thethreshold pressure.

Note that the example control and estimation routines included hereincan be used with various engine and/or vehicle system configurations.The control methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other engine hardware. The specificroutines described herein may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example embodiments described herein, butis provided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the engine control system, where thedescribed actions are carried out by executing the instructions in asystem including the various engine hardware components in combinationwith the electronic controller.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for an engine, comprising: estimating an exhaust gasrecirculation (EGR) mass flow based on a differential pressure sensoroutput when an engine load is below a threshold; estimating the EGR massflow based on an intake carbon dioxide sensor output when the engineload is above the threshold and independent of the differential pressuresensor output; and adjusting a spark timing based on the estimated EGRmass flow.
 2. The method of claim 1, wherein an EGR valve is in a fullyopen position when the engine load is above the threshold.
 3. The methodof claim 2, further comprising turning off EGR system diagnostics whenthe engine load is above the threshold.
 4. The method of claim 1,further comprising, during a first transition from a first load belowthe threshold to a second load above the threshold, estimating the EGRmass flow independent of the differential pressure sensor output andbased on the intake carbon dioxide sensor output.
 5. The method of claim4, wherein, during the transition, an EGR valve is ramped open to afully open position.
 6. The method of claim 1, further comprising,during a second transition from the second load above the threshold tothe first load below the threshold, estimating the EGR mass flowindependent of the differential pressure sensor output and based on theintake carbon dioxide sensor output.
 7. The method of claim 6, whereinan EGR valve is ramped closed from a fully open position to a less openposition, the less open position based on a desired EGR flow at thesecond load.
 8. The method of claim 1, wherein the EGR is a highpressure EGR (HP EGR).
 9. The method of claim 8, wherein the engine is aV-engine; and wherein the HP EGR is on only one bank of the engine.10-20. (canceled)